The present invention is directed to the measurement of small dimensions on the order of microns and below, and more particularly to methods for the fabrication and use of a standard having a dimension that deviates from a predetermined absolute value by a known amount.
Historically, technological progress has been intricately intertwined with the ability to accurately measure the lengths of objects and the times, or duration, of events. The more accurate (in absolute terms) the measurements of diminishing lengths and decreasing time have become, the more sophisticated areas of knowledge that have become accessible to advance technological progress.
The invention of the scanning electron microscope (SEM) has played a role in this advance because the SEM has provided an ability to "see", and sometimes to "see very well", objects which previously, even in the most sophisticated optical microscope, were either partially or completely unresolved. The SEM has enabled the extension of the limits of optical imaging (about several microns) down to well below 0.05 micron.
Technically, the invention of the SEM is founded on an innovative approach to imaging that uses a stream of energetic electrons rather than a stream of photons. Since the wavelength of electrons is much, much smaller than that of photons, the resolution, i.e. the ability to "see the object clearly", is dramatically improved. Indeed, a sub-micron feature is clearly imaged on an SEM's monitor by a 2-dimensional or 3-dimensional brightness-delineated contour.
The scanning electron microscope has, therefore, become an indispensable tool as a visual and measuring aid where small dimensions are critical, such as in the manufacture of micro-electronic devices, especially now that dimensional miniaturization has progressed into the micron and sub-micron domain.
The SEM also was a prototype of present day E-beam lithographic equipment. The extension to this application came from a realization that energetic electrons can be used, in addition to image generation, to "carve out" a desired pattern in electron-sensitive material, in which the presence or absence of exposure by electrons can be reproduced by subsequent development, as is done in optical lithography. With the availability of high-contrast and high-electron-sensitive organic materials, so-called resists, E-beam lithography has rapidly advanced into the micron and sub-micron domain.
However, the application of E-beam lithography to high-density patterns has enforced a realization that the fidelity of an E-beam exposed pattern to the designed one is governed not solely by the accuracy of beam positioning, but also by the intricate details of electron scattering which take place in the resist and in the supporting substrate during exposure. As insights into the pattern formation mechanism have been gained, it has been learned that two major components contribute to energy deposited at, or near, the pattern's edges and in this way define pattern fidelity. These two components are short-range low angle scattering (about 0.1 .mu.m or less), so called forward scattering, and long-range wide angle scattering (over several microns), so called back scattering.
Somewhat ironically, the influence of the back-scattering phenomenon on image formation in the SEM was acknowledged much later, when it was realized that edge-brightness loses its sharpness by broadening due to scattering electrons, when the scattering range of these electrons is comparable to the dimensions of features in the pattern.
Even more ironically, the same phenomenon of electron-scattering that generates the image infidelity for the actual pattern being imaged in the SEM, is also responsible for infidelity of the actual pattern to the designed one during E-beam exposure. The latter infidelity is not possible to assess because image infidelity in the SEM, even at the 1.mu.m level, is an unknown. Thus the problem to be addressed is one of how to access the magnitude of distortion that inherently affects the image infidelity in the SEM, in absolute terms.
Current E-beam lithographic equipment is recognized for its ability to produce a grating with the absolute accuracy of its periodicity not deviating more than several thousandths of a micron from the designed value. A grating, in the lithographic sense, is generated in a resist material by a periodic succession of exposed and un-exposed regions, so that after development, lines with intervening spaces are formed. The grating's period value, or pitch, is the distance between equivalent edges of neighboring lines or neighboring spaces. It is possible to achieve a highly accurate grating periodicity through accurate beam placement, as controlled by a high-precision laser-based interferometer incorporated into an E-beam writer, due to the symmetrical nature of electron-scattering-induced distortions. For the equivalent edges that determine pitch, these distortions are canceled, both during the E-beam patterning and during the SEM imaging. However, for the edges constituting a non-periodic element in a grating, i.e. a line or a space between lines, the distortions, by symmetry arguments, are doubled.
The presence of distortion generated by the interaction of energetic electrons with the material, under certain conditions, imposes limitations on the fidelity of the pattern that is written by the electron-beam lithographic technique and degrades the image fidelity of an object viewed in the SEM. The onset of these infidelities is empirically known to be operative when the size of the pattern's element and/or the dimensional proximity of those elements to each other become comparable with the range of the two major components in the electron-scattering phenomenon. These ranges, in turn, are known to be dependent on incident electron energy and the properties of the material in which the pattern is formed. While the range of each electron-scattering component is known on a quasi-quantitative level, the magnitude of electron-scattering-induced distortions generated during E-beam patterning and SEM imaging is known only on a qualitative level. As a result, a designed pattern's dimensions are fabricated larger than they should be, and the actual dimensions of the pattern are imaged in the SEM smaller than they actually are. Moreover, in SEM imaging, even in the absence of charging (an additional contributor to image infidelity), the magnitude of distortion is affected by the instability of the day-today performance of an SEM.
There are, in principle, only two ways to account for the magnitude of these distortions operative in SEM imaging. One way is to develop a theory that assuredly allows one to calculate the distortion's magnitude for a given material, at a given SEM setting and for a given pattern's dimension. The second way is to fabricate an artifact whose width is a priori pre-determined in absolute terms, as is the periodicity of a high-quality grating, and whose sensitivity to the operative distortions is not lost, as happens with the grating's period. With the aid of such an artifact, the magnitude of a distortion operative in the SEM imaging of this artifact can be determined experimentally as the difference between the known width and the width as measured in the SEM.
From the microelectronics manufacturing point of view, it appears that the most benefit can be derived from a dimensional standard of a resist feature supported by an appropriate substrate. In such a case, a scientifically-sound control of lithography, the most crucial step in device fabrication, can be achieved.